The development of sustainable and renewable energy conversion technologies is becoming increasingly important and economically more viable with respect to the current state of the fossil fuel-based economy. In particular, fuel cell technologies promise a combination of high energy conversion efficiency coupled with the potential for a large reduction in power source emissions. This stems from the fact that a fuel cell, unlike an internal combustion engine, is an electrochemical device capable of converting chemical energy in the form of hydrogen or alcohol-based fuels directly to electrical energy with little or no toxic discharge.
The development of a viable Proton Exchange Membrane Fuel Cell system (PEMFC) is currently being explored for a wide range of applications. Its successful integration into targeted industries, such as the automotive sector would have a favorable global environmental and economic impact. These fuel cells rely on a thin polymer membrane that functions as a solid ionic conductor moving electrochemically generated protons from the anode to the cathode. The membrane must also be impermeable to the fuel (typically hydrogen or methanol) thereby acting as a fuel separator preventing the unspent fuel from mixing with the sink-gas, i.e. oxygen in the air. The membrane must also act as an efficient insulator directing catalytically produced electrons through an external circuit enabling the power generated by the cell to be consumed by an external load. The physical and chemical attributes of the PEM must ensure: high proton conductivity, low electronic conductivity, low gas/fuel permeability, oxidative stability, thermal stability, hydrolytic stability, good mechanical properties, ease of processing and economical viability.
Nafion®, a perfluorinated ionomer initially developed for the chlor-alkali industry, is currently the material of choice for the PEMFC industry because of its commercial availability and demonstrated performance in fuel cells (e.g. excellent chemical and mechanical stability and high proton conductivity). However, Nafion® is quite costly to produce, has high fuel permeability to alcohol-based fuels such as methanol, and has a low operating temperature due to low mechanical integrity at higher temperatures and low to moderate glass transition temperature that are not ideally suited to many fuel cell applications. As a result, there is a great deal of interest in designing and developing new low cost polymer materials having superior properties targeted specifically to the PEMFC industry.
Substantial current research is aimed at designing and developing alternative polymer materials based on non-fluorinated or partially fluorinated polymeric systems. The majority of this work is based on non-fluorinated condensation polymers that contain ionic functionality randomly located along the polymer backbone. Generally these polymers can achieve suitable conductivity only at high ion exchange capacities (IEC) resulting in high water-up and large membrane dimensional changes that are unsuitable for practical PEM applications. It has been suggested that these sulfonated polymers are unable to form defined hydrophilic domains as the rigid polyaromatic backbone prevents co-continuous ionic clustering from occurring. Introduction of ionic pendant side chains or ionic blocks into these systems has shown promise in terms of materials performance, however the conductivity and membrane hydrodynamic properties typically remain lower relative to Nafion®.
On the other hand, Nafion® is a random copolymer comprised of a perfluorinated hydrophobic backbone that contains a number of short flexible pendant side chains with single hydrophilic sulfonic acid groups. It is this delicate balance of hydrophobic-hydrophilic properties within the material coupled with the increased mobility of the flexible ionic side chain that, in the hydrated form, leads to a co-continuous network of ionic channels through the material.
Microstructural analysis of Nafion® and other newly emerging materials has suggested that both chemical microstructure and nanoscale morphology of ionomer membranes can dictate material performance. Although the microstructure of Nafion® has been extensively examined, the exact structural morphology of Nafion® remains controversial and is not entirely understood. Furthermore, the limited number of chemical variations of Nafion® materials precludes a detailed systematic study linking polymer structure to material properties.
Microphase separation of block copolymers can be used to create well defined periodic microdomains of controlled morphology (e.g. cylinders, spheres, lamellae) on the nanoscale (10-100 nm). Microphase separation in block copolymers arises from the incompatibility between the different covalently linked blocks. The ability to control domain size and morphology results from the precise synthetic control over the relative block volume fractions and the polydispersity of each block. This typically limits the synthetic methodologies used for preparing these polymers to a limited number of monomers that can undergo living-type polymerizations.
Recently research has shown that comb polymers are also capable of creating unique and interesting nanoscale morphologies. Many of the fundamental rules that govern block copolymer microphase separation can be applied to comb polymers. This expands the possibilities of available synthetic methodologies to include some non-living polymerization techniques capable of producing functional polymers that can form microphase separated morphologies.
There remains a need for new polymeric materials that can be used in proton exchange membranes, particularly for the PEMFC industry.